The invention concerns a device for the attenuation of the movement of the tape tensioning motors in tape recorders during standstill of the tape drive roller.
In the case of the reel drives in magnetic tape devices, in general the regulation of the reel (or tape tensioning) motors via the motor current prevailed and not via the motor voltage, because thereby the torque which is proportional to the motor current is always under control. In the case of the regulation via the motor voltage, the current and thus also the torque for motor and reel could become damagingly high when the voltage applied to the motor acts in the same direction as the electromotive force of the motor.
The current regulation for the reel motors, however, has the disadvantage that in the case of standstill of the drive roller (capstan), the reels do not stand still. This leads to unnecessary wear at all friction points of the tape in the buffer chambers which as a rule are present in such devices and at the tape guide locations. The restlessness of the reel is to be attributed to the following causes:
If the tape loop of the magnetic tape is located exactly in the middle of the buffer chamber, then the current is regulated down in the associated reel motor by the regulating device to the value zero. The underpressure which prevails in the buffer chamber under the tape loop draws the tape loop, however, in the direction of the chamber floor. Conditioned by the unavoidable hysteresis of the loop position sensors, this migration of the tape loop from the middle of the buffer chamber is recognized only after traveling of a certain distance. Following this, the regulating unit attempts to correct the position of the tape loop again in such a manner that the tape loop is located in the middle of the buffer chamber. If the tape loop then again has arrived in the middle of the buffer chamber, the motor current is again regulated down to zero. As a result of the mass moment of inertia of the reel (with its coil of tape), of the reel motor and of the reel turntable, however, the reel and thus also the tape loop in the buffer chamber again moves away from the middle. Only when the adjacent sensor in the buffer chamber is exceeded or the reel is braked by the tape tension, again a current flows through the tape tensioning motor, which moves the loop again toward the middle of the buffer chamber. Within the hysteresis range, the motor is completely unattenuated and it thus moves constantly.